Electrical resonators are widely incorporated in modern electronic devices. For example, in wireless communications devices, radio frequency (RF) and microwave frequency resonators are used in filters, such as filters having electrically connected series and shunt resonators forming ladder and lattice structures. The filters may be included in a multiplexer, such as a duplexer, for example, connected between an antenna (or multiple antennas as in the case of multiple input, multiple output (MIMO) designs) and a transceiver for filtering received and transmitted signals, typically within a predetermined radio frequency band. Other types of multiplexers in which the filters may be included are diplexers, triplexers, quadplexers, quintplexers and the like, for example. The multiplexer interfaces between the antenna and each of various networks to enable transmitting signals on different transmit (uplink) frequencies and receiving signals on different receive (downlink) frequencies. The filters associated with the multiplexer typically include band pass filters, which provide passbands for passing various transmitted and received signals through relatively narrow frequency bands (blocking all signals with frequencies outside the passbands).
As will be appreciated, it is desirable to reduce the size of components of electronic devices. Many known filter technologies present a barrier to overall system miniaturization. With the need to reduce component size, a class of resonators based on the piezoelectric effect has emerged. In piezoelectric-based resonators, acoustic resonant modes are generated in the piezoelectric material. These acoustic waves are converted into electrical waves for use in electrical applications.
One type of piezoelectric resonator is a Bulk Acoustic Wave (BAW) resonator. The BAW resonator has the advantage of small size and lends itself to Integrated Circuit (IC) manufacturing tools and techniques. The BAW includes an acoustic stack. The acoustic stack includes, inter alia, a layer of piezoelectric material disposed between two electrodes. Acoustic waves achieve resonance across the acoustic stack, with the resonant frequency of the waves being determined by the materials in the acoustic stack.
Film bulk acoustic resonator (FBAR) filters are one type of BAW filters. FBAR technology is characterized by superior performance in terms of Q over frequency, effective coupling coefficient kt2, and precise frequency control. These FBAR performance characteristics translate to superior product performance in terms of (low) insertion loss, (satisfactory) roll off characteristics at filter edge, (optimum) isolation, and (highest) nonlinearity performance.
FBARs include a piezoelectric layer sandwiched between two metal electrodes, i.e., a top metal electrode and a bottom metal electrode. FBARs are placed above an air cavity, and rely on air cavity packaging technology to achieve required performance characteristics. As a result, air cavities below FBARs must necessarily be robust and must not interfere with the resonator frequency centering, Q values, or nonlinearities.
Known packaging for FBAR(s) may include a semiconductor microcap lid placed over the FBAR(s) and the above-noted air cavities formed below the FBAR(s). The microcap lid may be held above the FBAR(s) by posts that are formed from the same material as the microcap lid and that are integral with the microcap lid wafer. The microcap lids are a wafer-level silicon cap (microcap) micromachined from a high resistivity wafer.
Parasitic contributions from the microcap lid wafer/microcap lid degrade linear performance characteristics of the packaged FBAR product. The parasitic contributions arise from the bulk conductivity of the semiconductor (e.g., silicon material) used for the microcap lid, as well as surface capacitances and inversions, plus the charging and discharging of semiconductor trap states. Any solution to the parasitic contributions that provides high performance at a lower cost should not interfere with the air cavities below the FBAR, nor adversely affect the frequency centering, Q values, or nonlinearities.
Additionally, gold thermocompression bonds may be used to hold the posts to the substrate that includes the air cavity below the FBAR. The posts are aligned with the gold thermocompression bonds to affix the microcap lid wafer over the FBAR device wafer. The gold thermocompression bond is expensive. At least in part to due to this expense, the size of the area in which the posts are attached to the substrate is limited, and the gold thermocompression bond is used only around a perimeter of the posts. Additionally, the gold thermocompression bond is used at high-pressure/high-temperature which poses a potential obstacle to scaling FBARs to wafers larger than, for example, 200 millimeters. Moreover, the gold thermocompression bonds are IR-based rather than lithography based
What is needed are package structures that overcome at least the shortcomings of known package structures described above.